Very very nice, this! I love Harry being too scared of Severus' snark to make himself vulnerable -- I've always had a weakness for a Harry who's so gunshy that Severus has to be the one to catch him. Harry has, after all, seemed very much at home with the whole 'fool me once, shame on me; fool me twice, shame on you' mentality.
The critter twist was lovely, and very well played. I did kind of guess who'd sent it though, by the initial colouring. Still, that didn't diminish the sell.